It is known that graphic and photographic techniques, including digital rendering, are undergoing deep changes and are being developed by abandoning prints made by way of traditional optical systems and increasingly focusing on inkjet printing technology from digital files on sheets or rolls providing substrates having the most varied features and dimensions.
The individual images are obtained by cutting processes whereby the substrates are introduced into automatic cutting devices having cutting units configured to make cuts in a longitudinal direction, parallel to a feeding direction, as well as in a transverse direction perpendicular to the feeding direction.
The longitudinal cutting units of these automatic cutting devices are typically restrained to a transverse bar along which they are locked at predetermined positions corresponding to the longitudinal edges of the images printed on the substrate. At least one transverse cutting unit is instead movable along a cross bar, for example by way of an electric motor and a toothed transmission belt.
These cutting devices may advantageously be provided with means for correcting alignment errors of the substrate with respect to the feeding direction and to the transverse direction along which the longitudinal and transverse cutting units operate.
To this aim, suitable cutting marks that stretch out in the longitudinal, or feeding, direction and in the transverse direction are printed on the substrate close to the images and parallel to their edges. The cutting marks consist of one or more parallel black bands that indicate the position and orientation of the images printed on the substrate and therefore serve as a reference for the cutting operations.
An example of such a substrate is described by U.S. Pat. No. 6,536,892 B1, according to which the images and the transverse and longitudinal cutting marks are printed together with a same printing head.
The correcting means of the cutting devices typically include optical units, such as e.g. reflective optical cells, that are operably connected to a control system in turn comprising a microprocessor provided with a suitable control program.
When a printed substrate is made to advance in the longitudinal or feeding direction, the optical units focus the cutting marks, check their position, and whenever misalignments of the images printed on the substrate are detected in the feeding direction and/or in the transverse direction, the control system operates suitable actuators and/or motorized rollers that correct the position of the bars or guides on which the cutting units are mounted.
Patent EP 1883510 B1 in the Applicant's name describes an automatic cutting method and an automatic cutting device according to the preamble of the independent claim 1 and the independent claim 4, respectively.
The aforementioned automatic cutting devices with the related correcting means have been developed for the so-called “large format” printers, meaning that they can print images on substrates having a width of 1600 mm or larger. However, applications for smaller format printers are more and more required.
In the case of images printed on large format substrates, the cutting marks may generally be clearly distinguished from the printed images, so that the optical units of the automatic correcting means operate substantially free from alignment and/or cutting errors.
On the contrary, in the case of lower-size substrates, the cutting marks may be confused with portions and/or details of the printed images, as well as with text elements, due to their lower scale. Hence, there is a possibility that the optical units of the correcting means make reading errors and, consequently, it is likely that the correcting means intervene only partially or not at all.
This problem essentially concerns the transverse cutting marks, which are printed between consecutive images in the longitudinal or feeding direction, whereas no problem substantially exists as far as the longitudinal cutting marks are concerned, because these are continuous lines extending over the whole length of a substrate in the feeding direction.